1. Field of the Invention
The present invention relates to a surge current detection device provided in, e.g., a protector for insulating (i.e., protecting) a piece of equipment such as a communication circuit or a power supply circuit from a surge current, for example lightning, entering a conductor such as a communication wire or a power supply wire connected to the piece of equipment, to detect a state of the entrance of the surge current, such as a magnitude thereof, and specifically relates to a surge current detection device that upon entrance of a surge current, detects a state of the entrance of the surge current, using a magnetic field generated on a conductor.
2. Description of the Related Art
In recent years, with improvement in quality of pieces of equipment such as communication circuits and power supply circuits, there is a trend of increasing a number of cases in which devices are damaged by, e.g., lightning. For example, where lightning strikes around a facility, surge current flows from the site where lightning struck to the peripheral area. If the surge current enters the inside of the facility, the pieces of equipment in the facility may be damaged. Therefore, surge current detection devices that detect entrance of surge current are required.
Also, surge protection devices (SPD), which are protectors, are provided in facilities in order to prevent pieces of equipment from being damaged. In such protectors that serve to protect the pieces of equipment in the facilities from lightning surge current such as lightning, protective devices, which are lightning protection devices such as lightning protection tubes (i.e., arresters) and/or varisters included therein may also be damaged, and therefore, surge current detection devices that detect entrance of surge current are also required for protectors as in the facilities to be protected.
Protectors, in particular, e.g., varisters and arresters, which are internal protective devices, are devices that are deteriorated by application of surge current or surge voltage. As a result of the deterioration, the functions that the devices originally have may deteriorate, and thus, it is important to change a deteriorated protector promptly.
Examples of surge current detection devices, which are related art attempting to solve such problems, are disclosed in Japanese Patent Laid-Open Nos. 2005-150657, 2006-244889 and 2007-242569.
In a surge current detection device described in Japanese Patent Laid-Open No. 2005-150657, when a surge current generated by lightning flows in a conductor, a color of a heat-sensitive material is changed using Joule heat generated by the surge current to determine deterioration of a protective device such as a varister. In other words, a heat-sensitive material is set in a protective device such as a varister, and upon a surge current flowing and the protective device being activated, Joule heat is generated and changes the color of the heat-sensitive material.
A surge current detection device described in each of Japanese Patent Laid-Open Nos. 2006-244889 and 2007-242569, as in the aforementioned case, employs a determination method using Joule heat, which determines deterioration using shrinkage of a metal of a heat-shrinkable material. For example, the surge current detection device has a mechanism that upon shrinkage of a metal shrinkable material by Joule heat generated by a surge current, makes a display portion that has been shielded by the metal shrinkable material be exposed so that the display portion can be viewed, to perform determination.
Furthermore, in a technical field that is different from that of protectors, the art of magnetic display medium enabling visual check of content of information in a card is described in Japanese Patent Laid-Open No. 5-16578.
The magnetic display medium described in Japanese Patent Laid-Open No. 5-16578 includes an information storage portion and a magnetic display portion on a base. The information storage portion includes a magnetic tape or an integrated circuit (IC) memory. The magnetic display portion includes a substrate, and a recording layer containing microcapsules provided by coating on the substrate directly or via an intermediate layer. The microcapsule contains a liquid and magnetic powders (magnetic particles) that float in the liquid and are sensitive to a magnetic field, and based on information stored in the information storage portion, viewable information can be recorded on and erased from the recording layer in the magnetic display portion.
Also, a technique relating to a microcapsule is disclosed in Japanese Patent Laid-Open No. 11-76801.
However, the related art surge current detection devices described in Japanese Patent Laid-Open Nos. 2005-150657, 2006-244889 and 2007-242569 have problems (a) and (b) described below.
(a) In the related art devices, deterioration of a protective device such as a varister or an arrester is determined using Joule heat generated by a surge current flowing in a conductor. A conductor in which a surge current flows needs to make the surge current flow quickly, and thus, needs the resistance value to be small. However, a surge current flows in a short period of time, and only a small amount of Joule heat is generated, resulting in a failure to provide favorable deterioration detection sensitivity.(b) The device using a metal shrinkable material, etc., is a mechanical structure and thus, takes a lot of trouble in attaching the device to a conductor, and furthermore, is difficult to achieve reduction in size, weight and cost.
As described above, the related art surge current detection devices have the problems of a failure to provide favorable deterioration detection sensitivity, a lot of trouble taken in attaching the device to a conductor, and the difficulty of reduction in size, weight and cost.
In order to solve such problems, for example, a recording layer containing microcapsules, which is described in, e.g., Japanese Patent Laid-Open Nos. 5-16578 and 11-76801, may be applied to the related art to detect a surge current. However, a surge current such as lightning flows in an extremely short period of time, and thus, it is difficult to provide improved detection sensitivity.